The multidrug resistant P-glycoprotein, a 170-kDa plasma membrane protein encoded by the mammalian multidrug resistance gene (MDR), functions as an energy-dependent efflux pump of many of the most potent chemotherapeutic drugs in cancer treatment. Overexpression of the human MDRI gene is associated with resistance to a broad spectrum of chemotherapeutic agents. We are synthesizing and characterizing novel hexakis(R-isonitrile) technetium(I) and rhenium(I) complexes, lipophilic cationic metallopharmeuticals, as substrates for the MDR1 gene product. We are also synthesizing organotechnetium complexes to image human tumors in breast, lung and other cancers. Chemical analysis of these materials is criticially dependent on access tomass spectrometry.